1. Field of the Invention
The present invention relates to a toner for developing an electrostatic charge image, which is excellent in various characteristics including a charging property and is suitably used during formation of an image using an electrophotographic method, a method for efficiently producing the toner for developing an electrostatic charge image, and an image formation method using the toner for developing an electrostatic charge image.
2. Description of the Related Art
Methods for visualizing image information via an electrostatic charge image, such as an electrophotographic method, are widely used in various fields. In the electrophotographic method, an electrostatic charge image is formed on a photoreceptor via a charging process, an exposure process, etc. The electrostatic charge image is developed with a developer containing toner particles, and visualized via a transfer step, a fixing process, etc.
As the developer, two-component-type image developers containing toner particles and carrier particles, and one-component-type developers containing magnetic toner particles or nonmagnetic toner particles are known. Toner particles in the developer are usually produced in a kneading and pulverizing method. In the kneading and pulverizing method, a thermoplastic resin is melted and kneaded with a pigment, a charge controller, and a mold release agent such as a wax. After cooling, the melted and kneaded product is finely pulverized and classified to produce desired toner particles. In order to improve the flowability and cleaning property of the toner particles produced by the kneading and pulverizing method, inorganic and/or organic fine particles can be further added to the surface thereof as needed.
Toner particles produced in the kneading and pulverizing method usually have an amorphous shape without a homogeneous surface composition. Although the shape and surface composition of toner particles change slightly depending upon the pulverizability of the used material and conditions of the pulverizing process, it is difficult to intentionally control these elements to a desired degree. In addition, in the case of toner particles produced in the kneading and pulverizing method with a material with a particularly high pulverizability, due to mechanical forces in the developing device such as shearing force, it is often the case that the particles are pulverized still more finely or the shape thereof is altered. As a consequence, problems occur in the case of the two-component-type developer, the pulverized toner particles adhere to the carrier surface so that the charge deterioration of the developer is accelerated, and in the case of the one-component-type developer, the particle size distribution is expanded so that the pulverized toner particles scatter or the developing property is lowered according to the change of the toner shape, resulting in a deteriorated image quality.
In a case in which the toner particles have an amorphous shape, there is a problem that, even though a flowability aid is added, the flowability is insufficient and the fine particles of the flowability aid are moved to the concave portions of the toner particles to be buried therein during operation due to mechanical force such as shearing force, and thus flowability decreases over time or the developing property, transfer property, and cleaning property deteriorate. Furthermore, there is a problem that, by recycling the toner through recollection and cleaning treatment to return to the developer, it tends to deteriorate image quality. In order to prevent these problems, further increase of the amount of the flowability auxiliary agent can be considered; however, this involves problems in that generation of spots on the photoreceptor and particle scattering of the flowability auxiliary agent occur.
On the other hand, in a case of a toner containing a mold release agent such as a wax, the mold release agent may be exposed on the toner particle surface depending upon the combination with a thermoplastic resin. Particularly in the case of a toner combining a resin applied with elasticity by a high molecular weight component not easily pulverized and a vulnerable wax such as polyethylene, polyethylene exposure on the toner particle surface is often observed. Although such a toner has an advantageous mold releasing property at fixing or cleaning of untransferred toner on the photoreceptor, there is a problem since polyethylene on the surface of the toner particles easily fall off toner particles due to the mechanical force in the developing device such as shearing force and transfer to the developing roller, the photoreceptor, the carrier, etc., causing dirt that decreases the reliability of the developer.
Under such circumstances, nowadays, as a means for producing a toner whose particle shape and the surface composition are intentionally controlled, an emulsion polymerization aggregation method is proposed in Japanese Patent Application Laid-Open (JP-A) Nos. 63-282752 and 6-250439. The emulsion polymerization aggregation method is for obtaining toner particles by preparing a resin dispersion by emulsion polymerization and a colorant dispersion where a colorant is dispersed in a solvent, mixing for forming aggregative particles corresponding to the toner particle size, and heating for fusing. According to the emulsion polymerization aggregation method, the toner shape can be optionally controlled from amorphous to spherical by the selection of the heating temperature condition.
However, in the case of the emulsion polymerization aggregation method, since aggregative particles in a homogeneous mixing state are fused, the composition of the toner is homogeneous from the inside to the surface, and thus it is difficult to intentionally control the structure and composition of the toner particle surface. Particularly in the case the aggregative particles contain a mold release agent, the mold release agent exists on the toner particle surface after fusing so that filming generation and burial of the external additive used for the sake of flowability inside the toner may occur.
In order to maintain and pursue stable toner performance under various mechanical stresses in an electrophotography process, it is necessary to constrain the exposure of a mold release agent on the toner particle surface, to improve the surface hardness of the toner particle, and to further improve the smoothness of the toner particle surface. Although the mold release agent may cause various problems if it is exposed on the toner particle surface, it is preferable that it be near the toner particle surface in consideration of the toner performance at fixing.
Recently, owing to the need for higher image quality, particularly in color image formation, toners of a smaller size have been developed for realizing finer images. However, with conventional toner particle distribution, merely with a smaller size, it is difficult to simultaneously realize both high image quality and high reliability due to significant problems of dirt on a carrier or a photoreceptor and toner scattering due to finer toner particles. In order to realize high image quality and high reliability at the same time, a sharper toner particle distribution and a smaller particle size are needed.